1. Technical Field
The present invention relates to a contactless power transfer system and control method thereof that supply power mainly across a space, utilizing magnetic coupling in a contactless condition between coils.
2. Related Art
A contactless power transfer system supplies power to a load utilizing magnetic coupling between coils caused by electromagnetic induction. The principle thereof is that a sort of transformer is formed by magnetically coupling plural coils across a space, and power is supplied and received utilizing the electromagnetic induction between the coils.
For example, by disposing a primary side coil corresponding to a power supply source in a rail form as a feeder wire, configuring a moving body by integrating a secondary side coil and power receiving circuit, and causing the primary side coil and secondary side coil to oppose each other, it is possible to contactlessly transfer power to the moving body moving along the feeder wire.
Herein, FIG. 27 shows a heretofore known technology of a contactless power transfer system described in JP-A-2002-354711 (especially, paragraphs [0028] to [0031] and to [0045], FIGS. 1, 6, and the like). In FIG. 27, a primary side feeder wire 110 acting as a coil is connected to either end of a high frequency power source 100. A power receiving coil 120 is magnetically coupled to the primary side feeder wire 110, and the primary side feeder wire 110 and power receiving coil 120 configure one kind of transformer.
Both ends of the power receiving coil 120 are connected to alternating current terminals of a full-wave rectifier circuit 10 via a resonant capacitor C. Herein, the power receiving coil 120 and resonant capacitor C configure a series resonance circuit.
The full-wave rectifier circuit 10 is configured by bridge connecting diodes Du, Dv, Dx, and Dy.
A constant voltage control circuit 20, which controls in such a way that the direct current output voltage of the full-wave rectifier circuit 10 is of a reference voltage value, is connected to direct current terminals of the full-wave rectifier circuit 10. The constant voltage control circuit 20 is configured of a boost chopper circuit formed from, for example, a reactor L1, a diode D1, a smoothing capacitor C0, and a semiconductor switch SW1. Also, a load R is connected to either end of the smoothing capacitor C0.
A control device for switching the semiconductor switch SW1 is omitted from FIG. 27.
With the heretofore known technology of FIG. 27, a high frequency current is caused to flow along the primary side feeder wire 110 by the high frequency power source 100, and the high frequency power supplied is input into the full-wave rectifier circuit 10 via the power receiving coil 120, and converted into direct current power.
Generally, with this kind of contactless power transfer system, the voltage induced in the power receiving coil 120 changes due to a change in length of the gap between the primary side feeder wire 110 and power receiving coil 120, and due to positional deviations of the two, because of which the direct current output voltage of the full-wave rectifier circuit 10 fluctuates. The characteristics of the load R are also a cause of the direct current output voltage of the full-wave rectifier circuit 10 fluctuating.
For this reason, with the heretofore known technology of FIG. 27, the direct current output voltage of the full-wave rectifier circuit 10 is controlled to a constant value by the constant voltage control circuit 20.
For a contactless power transfer system, the higher the frequency of the current supplied via the coil, the lower the exciting inductance needed for carrying out a power transmission, and it is possible to miniaturize the coil and a core disposed in the periphery thereof. However, for a power converter configuring a high frequency power source device or a power receiving circuit, as the frequency of the current flowing through the circuit increases, the switching loss of the semiconductor switch increases, and the power transfer efficiency decreases, meaning that it is common to set the frequency of the contactlessly fed power between a few kilohertz and a few tens of kilohertz.
The contactless power transfer system shown in FIG. 27, and in particular the power receiving circuit after the resonant capacitor C, has the following problems.
1. As the power receiving circuit is configured of the full-wave rectifier circuit 10 and constant voltage control circuit 20, the circuit as a whole increases in size, which leads to an increase in installation space and an increase in cost.
2. As loss also occurs in the reactor L1, semiconductor switch SW1, and diode D1, in addition to in the diodes Du, Dv, Dx, and Dy of the full-wave rectifier circuit 10, these losses cause a decrease in power transfer efficiency.